The present invention is directed generally to a process for the application of organic polymer coatings to metal substrates. More specifically, the present invention relates to a multiple stage autodeposition of organic polymer coatings on metal substrates wherein the coated substrate as good barrier properties, adhesion, hardness and corrosion resistance.
The use of organic polymers as coatings on metal surfaces has been practiced on a commercial scale for a number of years. One such method is referred to as the electrocoat or electrodeposit process which is either an anodic or cathodic process whereby an electrical current of high potential is passed through the coating system causing the composition to coagulate on the surface of the article being coated. However, this process is disadvantageous in that it employs a high energy input of electricity. Further, it is difficult to coat the edges and corners of complex metal articles employing the electrodeposit process. The anodic electrodeposit process is further disadvantageous in that serious damage to the substrate can result from the oxidation reactions which occur when the substrate is the anode.
The dip coating process is a process in which the metal substrate for coating is immersed in the coating bath for a specified time and baked to cure the coating. This process is superior to the electrodeposition process in its ability to coat corners, edges and complex shapes. The dip coating process, however, is disadvantageous in that the coating on the substrate is thin, thus the coated substrate usually has poor corrosion resistance. Furthermore, due to the thin coating, pinholes readily form providing a pathway to the metal substrate for oxygen, water and various corrosion promoting ions.
The autodeposition method is a process in which an article is immersed in an acidic coating bath composed of organic film-forming material, water, hydrogen ion, oxidizing agent, and an anion and then the article is withdrawn and optionally rinsed in a solution containing chromium to improve the corrosion resistance of the article over the dip coating process. This type of method is disclosed in U.S. Pat. No. 3,585,084 to Steinbrecher et al. This method is disadvantageous in that there are environmental and pollution problems involved in disposing the hazardous chromium containing waste solutions, while corrosion resistance in the absence of the chromium rinse is poor.
Multiple stage autodepositions have been disclosed for increasing the thickness of the coating but not for improving the corrosion resistance without increasing the thickness of the coating. Multiple stage autodepositions for increasing the thickness of the coatings is disadvantageous in that some organic coating materials have poor intercoat adhesion and thus do not readily adhere to themselves causing peeling and poor corrosion resistance.
It is therefore an object of this invention to provide a process for the multiple stage autodeposition of corrosion resistant organic polymer coatings on metal substrates.
It is another object of this invention to provide corrosion resistant organic polymer coatings, by a multiple stage autodeposition process, which are thin, hard, highly adherent and resistant to impact shock.
It is another object of this invention to provide a process for the multiple stage autodeposition of corrosion resistant organic polymer coatings in the absence of a chromium rinse treatment.
These and other objects, together with advantages over known methods shall become apparent from the specification that follows and are accomplished by the invention as hereinafter described and claimed.